Apr. 11, 2013
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Lufengosaurus seen in museum display at full size. / Phil Gilston, Nature

by Dan Vergano, USA TODAY

by Dan Vergano, USA TODAY

How did dinosaurs grow so big? By starting early, suggest paleontologists reporting Wednesday on the discovery of 190-million-year-old fossilized dinosaur eggshells and bones, among the oldest ever found.

The discovery, near Kunming in southern China, suggests that the dinosaurs grew fast, even inside the egg.

"This is the first time we can actually look and see how fast the dinosaurs were growing," says study lead author Robert Reisz of the University of Toronto Mississauga. About 200 tiny bones were found among the eggshell fragments. Less than an inch long, the bones show numerous young dinosaurs at the very earliest stage of development, his team reports in the journal Nature. All the fossils appear preserved in nests inundated by slow flooding in ancient times.

"This study may at long last shed light on the mystery about how some of the giant dinosaurs got so big," says paleontologist Kenneth Carpenter of Utah State University-Eastern in Price, Utah, who was not part of the discovery team. Taken together, findings can serve as a "road map," he says, to understanding embryo development in other dinosaurs. "What is also cool is that the bones suggest rapid growth," Carpenter says.

The baby dinosaurs likely belonged to a two-legged, short-armed dinosaur called Lufengosaurus (LOO-feng-oh-sor-uss), named after the part of China where they were first discovered 75 years ago. When grown, the species likely were more than 20 feet long, stretching their necks like giraffes to eat vegetation they raked in using sharp claws. They were also forerunners to the humongous four-legged, long-necked and long-tailed "sauropod" dinosaurs seen in museum displays.

At full size, an adult Lufengosaurus likely weighed almost 2 tons; two full grown skeletons were found near the eggshell site. The eggs were likely more than 3 inches long when they were flooded out of nearby nests and deposited in silt that became a layer of red sandstone less than 8 inches thick.

Thigh bones from the site show that even in the eggshell, the baby dinosaurs were growing longer and thicker rapidly. The baby Lufengosaurus grew much larger and more quickly in the egg than other kinds of dinosaurs, or than birds do today, say the study authors.

Using high-tech bone scans, the team found thickening of those baby dinosaurs thigh bones, which indicates they were flexing their muscles even in the egg, nudging bones to grow into their adult shapes. That represents the earliest known evidence for this kind of interplay between muscle and bone growth in animals. And the scans reveal signs of residue from "dinosaurian soft tissues," says the study, likely collagen in the bones.

"The eggs were likely first smothered in water and then mud, leaving the animals behind," Reisz says.